Manchester United's Juan Mata carries the ball back to the centre circle after scoring his side's first goal from the penalty spot during the UEFA Champions League Group B match at Old Trafford, Manchester.

Manchester United's Juan Mata carries the ball back to the centre circle after scoring his side's first goal from the penalty spot during the UEFA Champions League Group B match at Old Trafford, Manchester.

Manchester United's Wayne Rooney rues a missed chance during the UEFA Champions League Group B match at Old Trafford, Manchester

MANCHESTER, ENGLAND - SEPTEMBER 30: Daniel Caligiuri of VfL Wolfsburg goes between Juan Mata (L) and Luis Antonio Valencia of Manchester United (R) during the UEFA Champions League Group B match between Manchester United FC and VfL Wolfsburg at Old Trafford on September 30, 2015 in Manchester, United Kingdom. (Photo by Dean Mouhtaropoulos/Getty Images)

MANCHESTER, ENGLAND - SEPTEMBER 30: Max Kruse of VfL Wolfsburg is chased by Luis Antonio Valencia of Manchester United during the UEFA Champions League Group B match between Manchester United FC and VfL Wolfsburg at Old Trafford on September 30, 2015 in Manchester, United Kingdom. (Photo by Dean Mouhtaropoulos/Getty Images)

MANCHESTER, ENGLAND - SEPTEMBER 30: Ricardo Rodriguez of VfL Wolfsburg evades Wayne Rooney of Manchester United during the UEFA Champions League Group B match between Manchester United FC and VfL Wolfsburg at Old Trafford on September 30, 2015 in Manchester, United Kingdom. (Photo by Dean Mouhtaropoulos/Getty Images)

MANCHESTER, ENGLAND - SEPTEMBER 30: Juan Mata of Manchester United (8) scores their first and equalising goal from the penalty spot past goalkeeper Diego Benaglio of VfL Wolfsburg during the UEFA Champions League Group B match between Manchester United FC and VfL Wolfsburg at Old Trafford on September 30, 2015 in Manchester, United Kingdom. (Photo by Dean Mouhtaropoulos/Getty Images)

MANCHESTER, ENGLAND - SEPTEMBER 30: David De Gea of Manchester United celebrates as Juan Mata of Manchester United (not pictured) scores their first and equalising goal from the penalty spot during the UEFA Champions League Group B match between Manchester United FC and VfL Wolfsburg at Old Trafford on September 30, 2015 in Manchester, United Kingdom. (Photo by Dean Mouhtaropoulos/Getty Images)

Juan Mata was the star of the show as Manchester United got off the mark in the Champions League with a narrow win over Wolfsburg.

Daniel Caligiuri took advantage of some slack defending from the hosts to put Wolfsburg ahead in the fourth minute, but Mata won - and converted - a penalty to equalise 11 minutes before the break.

The Spaniard then backheeled a lovely pass straight into Chris Smalling's path and he slotted the ball past Diego Benaglio to give United a 2-1 win.

United looked anything but potential Champions League winners - as Bastian Schweinsteiger claimed they were on the eve of the match - but Louis van Gaal will not be too perturbed.

After losing their first Group B match to PSV earlier this month, the United boss knew this was a must-win encounter and he will be glad his team just about dragged themselves over the line against a disciplined and threatening German opponent.

After a wobbly start, United are now second in their group, level on three points with Wolfsburg, PSV and CSKA Moscow.

United attacked with intent from the off, but the hosts were stunned in the fourth minute when Wolfsburg took the lead.

Smalling, Morgan Schneiderlin and Daley Blind failed to dispossess Caligiuri in the right corner. He played the ball inside to Max Kruse, who returned the ball to the midfielder in the box and he jabbed the ball past David de Gea. The goal would not have stood had Antonio Valencia been in line with his fellow defenders.

United looked for an immediate reply, but struggled to find one. Dante flung himself into the path of a Mata curler and diverted the goal-bound shot over the bar.

Anthony Martial shrugged off his marker and negotiated his way past another before rifling a shot just wide.

Matteo Darmian put in a crucial block deep inside the home box and United immediately went on the counter. Martial turned his marker and squared to the unmarked Wayne Rooney but he planted the ball into the crowd.

Finally the nerves eased inside Old Trafford when Mata cut in from the right and swung in a cross which was impeded by Caligiuri's outstretched left arm.

Referee Viktor Kassai pointed to the spot and Mata slotted the ball into the bottom left-hand corner.

United had more composure about them in defence after Ashley Young came on for Valencia at half-time.

Wolfsburg looked for another early goal, but luckily for United, Darmian was on hand to deny Bas Dost an easy tap in.

United then put themselves in charge of the game in the 53rd minute. Smalling rolled the ball past Wolfsburg goalkeeper Benaglio, but Mata should take the lion's share of the credit for the goal. The Spaniard flicked a wayward pass over the Wolfsburg back four and into Smalling's path. The defender slid in and guided the ball past Benaglio with his right foot.

Caligiuri tested De Gea with a low shot as Wolfsburg looked for an equaliser. Nicklas Bendtner and Andre Schurrle came off the bench to add to the Germans' attacking threat.

The former Arsenal striker fluffed a good chance just after entering the pitch and Wolfsburg continued to apply pressure, but United hung on for their first Champions League win under Van Gaal.

And what was Van Gaal's analysis of events?

"It was very difficult because in the fourth minute, they already scored," the Dutchman told BT Sport Europe.

"We created a lot of chances but we didn't finish it. It was a lucky penalty, I think, of course it is a penalty but there are also referees who are not whistling for that. Then it is 1-1.

"But we created much more chances. Rooney and Depay could have scored also in the first half and then you finish the game."

Van Gaal seemed to acknowledge his Red Devils side made life difficult for themselves in the second half, despite sealing victory thanks to Chris Smalling.

"Then we have to continue, of course," he added. "Okay, we score at once but after that it was suffering. A lot of players are very tired because of the cumulation of a lot of matches but still, we didn't give much away.

"How many chances have we given away? Maybe the once chance. But the rest of the chances were threats and we defended (against that).

"But we have to keep the ball better in the second half because that was our problem."

The Spaniard, who won the spot-kick, said: "The manager made a list of penalty takers and I am the first right now, so I have confidence."

Match-winner Smalling added: "We started off sloppy and with that goal you give yourself a big battle to get back into the game, but after that we created a lot of chances and I think it was a deserved victory."

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